The typical approach to steam temperature control in a boiler/turbine installation is to operate at the maximum possible main steam temperature, so as to maximize system efficiency, while not exceeding the maximum metal temperatures allowed in the boiler and/or turbine or the maximum allowed rate of change of these temperatures. Such temperature control is generally accomplished through a combination of feedforward and feedback controls that utilize a combination of pressure, temperature, steam flow, and heat flow measurements to adjust the final superheat temperature, i.e., the main steam temperature. This adjustment usually involves varying the water flows through an attemperating spray valve into the secondary superheater section of the system or by varying the flue gas recirculation rate through the boiler. In any event, the system requires the establishment of a main steam temperature set point. Inasmuch as there is a wide variation in possible operating conditions for the boiler and since these control systems do not provide for the automatic reduction of this set point if the main steam temperature approaches the danger level, the main steam temperature set point is selected in a conservative manner so that the main steam temperature safety limit is not exceeded over the full range of boiler operating conditions and possible disturbances. The end result of having to utilize a conservative value for the main steam temperature is that the boiler/turbine installation does not operate at maximum efficiency.
Because of the foregoing, it has become desirable to develop a control system for a boiler/turbine installation which would permit the installation to operate at maximum efficiency over the full range of boiler operating conditions.